The regulation of mammalian cell growth is governed by both positive and negative signaling pathways. The retinoblastoma (Rb) gene encodes a family of nuclear phosphoproteins, termed p105-Rb, whose function is thought to be to limit or constrain cell proliferation. The mechanism by which Rb-mediated growth restraint is imposed is currently unknown. Furthermore, the proteins and biochemical signals that regulate pl05-Rb activity have yet to be characterized. Given the importance of Rb inactivation in the etiology of various human tumors and the widespread distribution of plO5-Rb in all mammalian cells , it is clear that plO5-Rb plays a central role in regulating normal cell growth. To determine the function of plO5-Rb and to define its contribution towards negative-growth regulation we propose to address the following specific aims: 1. Through biochemical and genetic means we will identify and clone genes encoding proteins that interact with plO5-Rb. These proteins will be characterized for their biochemical properties, distribution, and role in regulating or mediating Rb activity. 2. Through a series of molecular and biochemical experiments we will establish the mechanism by which pl05-Rb is able to mediate transcriptional repression of the human c-fos promoter.Additional genes that may be regulated by pl05-Rb will be identified and the mechanism(s) by which transcriptional repression is regulated will be determined.